Titaniferous materials are often subjected to chlorination, as chlorination is an efficient and economical way to obtain a high purity titanium source for making titanium alloys, titanium compounds, and especially pigmentary titanium dioxide.
Several processes have been described in the art for the chlorination of titaniferous materials. Such processes generally react a titanium-containing raw material such as rutile or ilmenite ore, with a chlorine-providing material and a carbon-containing reductant according to one or both of the following equations: EQU TiO.sub.2 +2Cl.sub.2 (g)+C(s).fwdarw.TiCl.sub.4 (g)+CO.sub.2 (g) EQU TiO.sub.2 +2Cl.sub.2 (g)+2C(s).fwdarw.TiCl.sub.4 (g)+2CO(g)
Iron is a common impurity in titaniferous raw materials, and most chlorination processes are effective for simultaneously chlorinating the Ti and Fe values of these raw materials as shown in the following reactions: EQU 2FeTiO.sub.3 +6Cl.sub.2 (g)+3C(s).fwdarw.2TiCl.sub.4 (g)+3CO.sub.2 (g)+2FeCl.sub.2 EQU FeTiO.sub.3 +3Cl.sub.2 (g)+3C(s).fwdarw.TiCl.sub.4 (g)+3CO(g)+FeCl.sub.2
Chlorination reactions are generally carried out at about 1000.degree. C., but can be carried out at any temperature in the range from about 400.degree. C. to about 2000.degree. C., using various carbon reductants and chlorine sources, including chlorine gas and chlorine-containing compounds. The titaniferous raw materials to be chlorinated can be preformed into briquets or the process can be conducted in a fluid bed using granular materials. When a fluid-bed process is used, generally the chlorine-providing material is supplied to the bottom of the bed and product titanium tetrachloride (TiCl.sub.4) is removed from the top. Fluidization is generally controlled such that the bed remains fluidized and yet fine, solid particulate materials are not carried out with the product.
Selective chlorination processes also exist and are designed to chlorinate only the Ti values or the Fe values of the raw material. A carbon reductant and a chlorine source are used and reaction temperatures are similar to non-selective processes. However, selective processes utilize a chlorine source consisting at least partially of iron chlorides, react the titaniferous raw materials in a dilute phase, react the titaniferous raw materials at a specially high temperature, or a combination of the above.
Titanium raw materials such as rutile and ilmenite ores also usually contain vanadium compounds as impurities which adversely affect the titanium products produced. For example, pigmentary TiO.sub.2 can tolerate only about 10 ppm. vanadium in the titanium tetrachloride from which the TiO.sub.2 is made without discoloration. Removal of such impurities has heretofore been a complicated and burdensome process because of the similarity between the chemical and physical characteristics of titanium compounds and vanadium compounds. For example, TiCl.sub.4 melts at -25.degree. C. and boils at 136.4.degree. C. and VCl.sub.4 melts at -28.degree. C. and boils at 148.5.degree. C. This parallelism of properties permeates a comparison of the compounds of these two elements. Therefore, in the conventional chlorination process the vanadium values in a titaniferous raw material react in substantially the same manner as the titanium values, and their respective chlorinated products have nearly identical chemical and physical properties. Accordingly, it is extremely difficult to separate the undesirable chlorinated vanadium values from the desirable titanium values. Fractional distillation, for example, will remove most impurities from TiCl.sub.4, but is ineffective for removing vanadium impurities.
Processes which are used commercially remove vanadium impurities from TiCl.sub.4 by refluxing with copper, treating with H.sub.2 S in the presence of a heavy metal soap, or treating with an alkali metal soap or oil to reduce vanadium impurities to a less volatile form. In each of these processes the treated TiCl.sub.4 is then subjected to a further distillation. However, the organic materials used tend to decompose and deposit sticky, adhering coatings on heat exchanger surfaces, pipes, and vessel walls. This causes shutdowns of the process and requires frequent maintenance of the equipment.
In accordance with this invention, a simple, efficient, and economical process has now been discovered for separating the vanadium values from chlorinated titaniferous materials. The process of this invention utilizes a high surface area carbon for reacting with the titaniferous materials during the chlorination process. The use of the high surface area carbonaceous material causes the vanadium values present in the titaniferous material to be reduced to a less volatile form so that they can be easily removed as a solid from the gaseous or liquid TiCl.sub.4 product.
One advantage of the present process is that it can be performed in existing equipment for the chlorination of titaniferous material. Another advantage is that it employs economical raw materials. Still another advantage is that the CO value of the tail gas produced is sufficiently enhanced such that said tail gases will support combustion and can be burned to effect complete conversion to CO.sub.2 and thus eliminate the pollution problem they previously created. These and other advantages will become more apparent in the "Detailed Description of the Invention".
Another advantage of this invention is in the surprising beneficial effect of the kind of reactive carbon used in the fluid bed reaction. I have found that a lignite or brown coal char which is a reactive carbon, has an unusual charactertistic in that the surface area of the char remains unchanged or actually increases during use providing still better combustibility of the tail gas than heretofore available. Also the greater the surface area of the carbonaceous material in the bed, the more efficient the chlorination reaction.
When treated anthracite is used in the conventional fluid-bed chlorination of a titaniferous ore, the carbon surface area decreases to a stable equilibrium value. Reactivity is directly related to surface area. If the "as prepared" surface area is high enough, the equilibrium surface area will also be high enough to obtain good chlorination results as shown in Table I below. If the initial surface area of the carbon is lower, the CO level will be lower, and the vanadium impurity level will be higher as shown in Table II below. If the vanadium level is greater than 10 ppm, organic additives must be added to crude liquid TiCl.sub.4 to assist in reducing the vanadium level below 10 ppm.
TABLE I ______________________________________ Using Steam Treated Anthracite Surface Area 481 m.sup.2 /g ______________________________________ Chlorination Time, hrs. 15.9 18.9 Bed Carbon Surface Area*, m.sup.2 /g 125 130 Average CO.sub.2 /CO Mole Ratio 0.03 0.02 Soluble V in Crude TiCl.sub.4, ppm -- 6 ______________________________________ *Method A
TABLE II ______________________________________ Using Steam Treated Anthracite Surface Area 160 m.sup.2 /g ______________________________________ Chlorination Time, Hrs. 2.9 9.3 16.7 Bed Carbon Surface Area*, m.sup.2 /g 37 21 21 Average CO.sub.2 /CO Mole Ratio 0.04 0.05 0.11 Soluble V in Crude TiCl.sub.4, ppm 68 151 424 ______________________________________ *Method A
Conventional fluid-bed chlorinations have been made using various carbons. Normally, low surface area carbons such as petroleum coke or bituminous coal char change very little in surface area during chlorination.
Contrary to the teachings of my prior U.S. Pat. Nos. 4,310,495 dated Jan. 12, 1982 and 4,329,322 dated May 11, 1982, instead of using a "high rank" coal char, e.g., anthracite, according to the present invention, I was surprised by the unexpected results that I got when a brown coal char was used. This material is derived from a low rank, lignitic ANSI/ASTM Class IV coal. The surface area of this carbon increased during chlorination as shown in Table III. Although the char had an "as prepared" surface area of only 30% of the treated anthracite in Table I, the char gave better chlorination results.
TABLE III ______________________________________ Using Australian Brown Coal Char Surface Area 147 m.sup.2 /g ______________________________________ Chlorination Time, Hrs. 4.0 13.6 20.2 Bed Carbon Surface Area*, m.sup.2 /g 189 180 218 Average CO.sub.2 /CO Mole Ratio 0.005 0.010 0.014 Soluble V in Crude TiCl.sub.4, ppm &lt;0.5 &lt;0.5 &lt;0.5 ______________________________________ *Method A
Brown coal char is available in large quantities in a size range suitable for fluid-bed chlorinators and at a cost that so far seems very attractive.
The relationship between carbonaceous source and surface area changes during chlorination was investigated further. I tried another ANSI/ASTM Class IV lignite-based carbon. As shown in Table IV, the surface area of this carbon also increased during chlorination. Very high CO levels and low vanadium impurity levels were observed as would be expected from the high equilibrium surface area of the bed carbon.
TABLE IV ______________________________________ Using a Lignite-Based Carbon Surface Area 625 m.sup.2 /g ______________________________________ Chlorination Time, Hrs. 3.0 10.9 15.0 27.5 Bed Carbon Surface Area*, 878 856 899 1005 m.sup.2 /g. Average CO.sub.2 /CO Mole Ratio 0.003 0.002 0.002 0.002 Soluble V in Crude TiCl.sub.4, &lt;0.3 &lt;0.3 &lt;0.3 &lt;0.3 ppm. ______________________________________ *Method A
The chemical and physical properties of reactive carbons are summarized in Table V.
TABLE V ______________________________________ Steam-Treated Australian Lignite- Anthracite Brown Coal Based #1 #2 Char Carbon ______________________________________ % C 84.5 80.2 91.9 80.0 % H 0.7 0.5 0.9 0.8 % Ash 11.9 17.6 2.3 12.0 % Volatile 0.8 1.1 5.6 6.6 Matter Nominal U.S. 6 .times. 50 6 .times. 50 8 .times. 35 12 .times. 20 Standard Sieve Size Surface Area, m.sup.2 /g Method A 160 481 147 625 ______________________________________
The only change that this invention contemplates from the prior processes is in the carbonaceous material used. The disclosures of the foregoing U.S. patents are incorporated in toto herein by reference as disclosures of the processes in which the discovery hereof may be used.
The best lignite or brown coal char of which I am aware for use in the processes of my prior patents is a brown coal char produced by Australian Char Pty. Ltd. and sold under the name "Auschar".
The preferred char has the following typical properties:
______________________________________ Physical Properties: Average size = 0.5 to 2.3 mm Specific Gravity = 1.2 Porosity = 33% Bulk Density = 640 kg/m.sup.3 Proximate Analysis of Dry Char: Ash = 2-3% Volatile Matter = 2-5% Fixed Carbon = 91-95% Surface Area Measurement: (by CO.sub.2 absorption at 0.degree. C.) about 750 m.sup.2 /g Ash Fusion Temperatures: In oxidizing atmosphere = 1460.degree. C. In reducing atmosphere = 1400.degree. C. Gross dry calorific value = 7900 kilocalories/ kilogram ______________________________________ NOTE: Other sizings are available to meet specific process needs.
Although Char is dry as it leaves the retort, it will pick up atmospheric moisture, and, after a time, may contain approximately 10% moisture. The figures quoted above are indicative of low volatile chars produced, and it should be noted that it is possible to vary the analysis to the extent of producing higher volatile materials when required.
______________________________________ PROXIMATE ANALYSIS OF DRY CHAR (TYPICAL) ______________________________________ Carbon = 94.5% Calcium = 0.08% Hydrogen = 1.1% Magnesium = 0.24% Nitrogen = 0.6% Aluminum = 0.06% Sulphur = 0.27% Silicon = 0.05% Iron = 0.33% Sodium = 0.11% ______________________________________ and traces of Chlorine, Sodium, Potassium, Phosphorous, Titanium and Copper?
For details of raw lignite or brown coal, reference may be had to Kirk and Othmer "Encyclopedia of Chemical Technology", Third Edition, Vol. 14, pages 313-343.
The chlorination processes of the present invention are to be distinguished from the post-chlorination vanadium removal treatment using activated carbon made from lignite disclosed in my U.S. Pat. No. 4,279,871. In this process a high surface area activated carbon made from lignite is entrained in the chlorinated gaseous product stream as it leaves the hot fluid bed. Vanadium impurity levels are vastly reduced by this treatment. In the present process, chlorination is done in a fluid bed composed of lignite or brown coal char and the ore.